You might be able to do math yourself, but there are many practical reasons why you wouldn't or couldn't do all sorts of math -- even simple math.

For example, how you find the median value of a billion random numbers without a computer? You wouldn't have time in your lifetime to do this, not to mention that you'd probably make a lot of mistakes despite the process being very simple in principle.

A computer could do this easily, and even were it to be a little off in its answer, that would be far preferable to devoting your entire lifetime to finding the answer yourself.

That's just one very simple example. It's not hard to find plenty more.